1. Field of the Invention
The present invention relates generally to a high-frequency oscillator. More particularly, the invention relates to a high-frequency oscillator of high output for generating plasma to be used in a plasma etching process.
2. Description of Relevant Art
Recently, as one of the most important techniques for the production of integrated circuits, there has been utilized an etching technique using plasma, in which a pressure-reduced gaseous body is caused to discharge by an electric field of high frequency to generate low-temperature plasma containing chemically active species (atom, molecule, ion). A surface of a wafer is thereby etched to be processed by taking advantage of the fact that a chemical reaction between the active species and the surface layer produces a substance of high vapor pressure.
For such etching technique, there has been employed a high-frequency oscillator, in which a high-frequency wave of 13.56 MHz is generated in a quartz oscillating circuit and amplified by means of a transistor or, when a large output of power is required, by a vacuum tube. From a small power output of 100 W, there is thereby obtained a high-frequency large-power output exceeding 1 kW.
In fabricating a high-output oscillator requiring large-power amplification, a vacuum tube is generally employed. However, the vacuum tube has as a basic shortcoming thereof a relatively short service life and, in normal use, requires readjustment or replacement after the lapse of only thousands of hours, thereby creating a major obstacle in the etching process.
On the other hand, although free of the service life problems of the vacuum tube, the transistor, or more generally the semiconductor device for amplification, has had attendant technical difficulties in the adaptation thereof as a high-frequency element for high voltage and high output, besides having a weak point of secondary breakdown, and has proven unsuitable for power amplification. The more recently developed MOS FET (metal-oxide semiconductor type field effect transistor), on the other hand, has put to practical use a high-performance solid-state element which can serve for high-frequency high output without the secondary breakdown attendant ordinary transistors, and is highly suitable for large-power usage.
The high-frequency oscillator employed in the art of plasma etching must meet various performance requirements, such as handling high-frequency waves, particularly of ten-odd mega-hertz, and large power outputs reaching a number of kilowatts, as well as generating stable sine waves of high accuracy and easily and widely varying the output power thereunder. However, there have been no proposed high-frequency oscillators sufficiently satisfying such requirements by employing an MOS FET adapted for large power.
Moreover, in an etching process in which a high-frequency voltage is applied to a gaseous body enclosed in a reactor, at certain levels of the high-frequency voltage a structural defect may be caused in a workpiece such as a silicon wafer being processed, due to the application manner of the high-frequency voltage. In this respect, a desideratum has developed for an effective measure for the application of high-frequency voltage.
The present invention effectively fulfills the aforesaid desideratum.